1. Field of the Invention
This invention relates to a drive system for an electromagnetically driven shutter which is electromagnetically opened and closed.
2. Description of the Prior Art
The driving force required for opening and closing the shutter of a camera generally has heretofore been derived from stored mechanical energy such as a spring force or the like irrespective as to whether the shutter is a lens shutter or a focal plane shutter. This has necessitated the use of a complex mechanism for driving a shutter with the stored mechanical energy. Accordingly, a complex mechanism has been also required for opening and closing the shutter. Such mechanisms have required the use of numerous component parts. In view of this, various kinds of electromagnetically driven shutters for producing a shutter driving force have recently been proposed. In these electromagnetically driven shutters, a driving force for opening and closing the shutter is electromagnetically generated by a drive mechanism. Therefore, the use of such a drive mechanism obviates the necessity of the type of mechanism required for a mechanical cocking action. Also, such an electromagnetically driven shutter permits replacement of all the mechanisms required for exposure control with electronic circuits. Thus, the electromagnetically driven shutter has many advantages over conventional mechanically driven shutters.
However, the structural arrangement of electromagnetically driven shutters has a number of drawbacks. For example, electromagnetically driven shutters in general use a moving coil type electromagnetic drive device having a permanent magnet and a moving coil. Between the pole piece and the yoke of the permanent magnet, is a bobbin on which a coil is wound. The bobbin is interlocked with the shutter. Therefore, when the coil is energized, an interaction between the current thus supplied and a magnetic flux generated by the permanent magnet between the yoke and the pole piece causes a force to be exerted on the bobbin in one direction to open the shutter. When the direction of the current supplied to the coil to energize it is reversed, the force is exerted on the bobbin in the reverse direction to close the shutter. For placing an electromagnetic drive device of this type within a camera, the size of the electromagnetic drive device is limited by a limited space available for accommodating it within the camera. Therefore, even where an alnico magnet is employed as permanent magnet, the magnetic flux generated in the gap between the yoke and the side face of the pole piece is 3000 to 5000 gauss. The electric current to be supplied to the coil is 0.5 to 1.0 A at the most, even with a single-3 alkaline battery used as battery. Further, in order to obtain stable current supply after the battery has been drained to some degree, the current becomes less than the aforementioned value.
Therefore, the driving force obtainable from an electromagnetic drive device is only several ten grams at the most. With such a driving force used for a shutter, it takes as much as 20 to 50 m sec. before the shutter is fully opened after the coil is energized. Then, for fully closing the shutter, a length of time 20 to 50 m sec. is also required. An example of the characteristic curves of such a shutter operation is illustrated by a curve a in FIG. 1 of the accompanying drawings. Furthermore, where an available space necessitates the use of a button type silver oxide or mercury cell as battery, the current available from such a battery is not more than several tens of m A. Therefore, in such a case, the length of time required for fully opening the shutter becomes still longer. To operate the shutter at a high speed of 1/500 or 1/1000 sec. with such an electromagnetic drive device, therefore, the shutter must be closed before it is fully opened. Then, the maximum aperture value becomes extremely small (less than f 32). A shutter characteristic curve of such a shutter operation is as represented by a curve b in FIG. 1. This tends to result in increasingly inconsistent of exposure values due to uneven cut of the aperture, etc. Also, diffraction might take place because of the extremely small aperture. Therefore, a shutter drive device of this type is not usable for a high speed shutter operation.
To solve this problem, U.S. Pat. No. 4,072,965 has disclosed a system for obtaining a driving force in which a capacitor is employed as current source for energizing the coil of an electromagnetic drive device; the capacitor is charged with a boosted voltage obtained by boosting the voltage of a battery; and the electric charge thus obtained is applied to the coil to obtain a driving force.
In this system, a driving current is obtained through a capacitor, so that a driving current which is several times as large as the driving current obtainable from a battery can be produced to permit a high speed shutter operation. However, in accordance with the system of this prior art, a large capacitance is required for accommodating the capacitor and this presents a problem with regard to an available space, etc. Assuming that a shutter is to be driven by the electric charge of the capacitor up to a low speed time of about 1/15 sec., for example, and assuming that the electric current I to be supplied to the coil is about 1 A, the quantity of electric charge Q of the capacity to required for flowing this current for a period of time t.sub.1, 60 m sec. is: EQU Q=IT=1.times.60.times.10.sup.-3 =0.06 (coulomb)
Then, assuming that the capacitor is charged with a voltage of 20 V, the capacitance C of the capacitor required for obtaining this much electric charge is: ##EQU1##
To have that high a capacitance, the capacitor must be of a size measuring about 20 to 30 mm in diameter and 30 to 60 mm in length. Then, such a large size makes it very difficult to place the capacitor within a limited space available in a camera.
The present invention is directed to the elimination of the above stated drawbacks of the prior art.